05 Civic Coolant flows to overflow, doesn't come back

I have an 05 Civic Ex, and noticed a while back that the coolant was low. I kept thinking that I had a leak somewhere, but have so far not been able to locate one. I would top it off every week or two, and then all would be fine for a while.
After doing this for a while, I noticed that the overflow tank was getting full. It seems that coolant is flowing into the overflow, but is not flowing back into the radiator once it cools down. I replaced the cap, hoping it was simple. That didn't help. I checked the hose that leads to the overflow. Coolant flows easily both directions.

I did notice that once the engine is completely cooled down, the cap still holds vacuum. When I take it off, there is a very noticeable sucking of air into the radiator.

It has never overheated, it's not leaking coolant anywhere that I can see, the exhaust doesn't seem to be smoking at all. Any suggestions as to where I should start?

Sounds like its the rad cap. The rubber can swell over time and cause this.

I just replaced the cap about two weeks ago. It is still having the same issue.

Did you replace the cap with a Honda product or generic cap from an auto parts store?